Parameters

Refers to the command-queue in which the read command will be
queued. command_queue and buffer
must be created with the same OpenCL context.

buffer

Refers to a valid buffer object.

blocking_read

Indicates if the read operations are
blocking or non-blocking.

If blocking_read is CL_TRUE
i.e. the read command is blocking, clEnqueueReadBuffer
does not return until the buffer data has been read and copied into memory
pointed to by ptr.

If blocking_read is CL_FALSE
i.e. the read command is non-blocking,
clEnqueueReadBuffer queues a
non-blocking read command and returns. The contents of
the buffer that ptr points to cannot be used until the
read command has completed. The event argument returns
an event object which can be used to query the execution status of the
read command. When the read command has completed, the contents of the
buffer that ptr points to can be used by the application.

offset

The offset in bytes in the buffer object to read from.

size

The size in bytes of data being read.

ptr

The pointer to buffer in host memory where data is to be read into.

event_wait_list
,
num_events_in_wait_list

event_wait_list
and num_events_in_wait_list specify events that
need to complete before this particular command can be executed.
If event_wait_list is NULL, then this particular command
does not wait on any event to complete. If event_wait_list
is NULL, num_events_in_wait_list must be 0. If
event_wait_list is not NULL, the list of events
pointed to by event_wait_list must be valid
and num_events_in_wait_list must be greater
than 0. The events specified in event_wait_list
act as synchronization points. The context associated with events in
event_wait_list and command_queue must
be the same. The memory associated with event_wait_list
can be reused or freed after the function returns.

event

Returns an event object that identifies
this particular read command and can be used to query or queue a wait
for this particular command to complete. event can be
NULL in which case it will not be possible for the application to query
the status of this command or queue a wait for this command to complete.
If the event_wait_list and the event arguments are
not NULL, the event argument should not refer to an
element of the event_wait_list array.

Notes

Calling clEnqueueReadBuffer to read a region of the buffer object
with the ptr argument value set to host_ptr
+ offset, where host_ptr is a pointer to
the memory region specified when the buffer object being read is created with
CL_MEM_USE_HOST_PTR, must meet the following requirements in
order to avoid undefined behavior:

All commands that use this buffer object or a memory object (buffer or image)
created from this buffer object have finished execution before the read command
begins execution.

The buffer object or memory objects created from this buffer object are not mapped.

The buffer object or memory objects created from this buffer object are not used
by any command-queue until the read command has finished execution.

Errors

clEnqueueReadBuffer returns CL_SUCCESS if
the function is executed successfully. Otherwise, it returns one of the following errors:

CL_INVALID_COMMAND_QUEUE if command_queue
is not a valid command-queue.

CL_INVALID_CONTEXT if the context associated with
command_queue and buffer are not the same
or if the context associated with command_queue and events in
event_wait_list are not the same.

CL_INVALID_MEM_OBJECT if buffer is not
a valid buffer object.

CL_INVALID_VALUE if the region being read specified by
(offset, size) is out of bounds or if
ptr is a NULL value or if size is 0.

CL_INVALID_EVENT_WAIT_LIST
if event_wait_list is NULL and
num_events_in_wait_list greater than
0, or event_wait_list is not NULL and
num_events_in_wait_list is 0, or if event objects in
event_wait_list are not valid events.

CL_MISALIGNED_SUB_BUFFER_OFFSET if
buffer is a sub-buffer object and offset
specified when the sub-buffer object is created is not aligned to
CL_DEVICE_MEM_BASE_ADDR_ALIGN value for device associated
with queue.

CL_EXEC_STATUS_ERROR_FOR_EVENTS_IN_WAIT_LIST if the read
and write operations are blocking and the execution status of any of the events
in event_wait_list is a negative integer value.

CL_MEM_OBJECT_ALLOCATION_FAILURE if there is a failure to
allocate memory for data store associated with buffer.

CL_INVALID_OPERATION if
clEnqueueReadBuffer is called on buffer
which has been created with CL_MEM_HOST_WRITE_ONLY or
CL_MEM_HOST_NO_ACCESS.

CL_OUT_OF_RESOURCES if there is a failure to allocate
resources required by the OpenCL implementation on the device.

CL_OUT_OF_HOST_MEMORY if there is a failure to allocate
resources required by the OpenCL implementation on the host.